This application claims the priority of Korean Patent Application No. 2004-65880, filed on Aug. 20, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a lens system and a portable device, and more particularly, to a lens system that prevents redness from occurring at the center region of an image as the lens system is miniaturized, and a portable device employing the lens system.
2. Description of the Related Art
Generally, image pick-up devices such as charge coupled devices (CCDs) or complementary metal oxide semiconductors (CMOS) used in, for example, cameras, react to light even in the near-infrared region beginning at a wavelength of about 700 nm and in the infrared region. However, light in the near-infrared and infrared regions causes crosstalk that reduces a signal-to-noise ratio of an image pick-up device and thereby deteriorates the device's reproduction ability. In order to prevent such crosstalk, an infrared cut filter that blocks light in the near-infrared and infrared regions is installed in lens systems implemented in conventional cameras.
For example, in a conventional camera, an infrared cut filter 4 is installed between an object and a lens system, which includes a first lens 1, a second lens 2, and a third lens 3, as illustrated in FIG. 1. Alternatively, the infrared cut filter 4 can be interposed between the lens system, including the first lens 1, the second lens 2, and the third lens 3, and an image pick-up device 5 that receives light transmitted through the lens system, as illustrated in FIG. 2.
However, when placing the infrared cut filter 4 between the object and the lens system as illustrated in FIG. 1, the outer diameter of the infrared cut filter 4 is increased, thereby increasing the cost of the portable device. Also, as will be described later, increase in an angle at which light is incident on the infrared cut filter 4 is accompanied by increase in a telecentric angle, which creates reddish noise around the center of an image or bluish noise around the edges of an image. In addition, when disposing the infrared cut filter 4 between the lens system and the image pick-up device 5 as illustrated in FIG. 2, a back focal length (BFL) increases, which increases the size and cost of the image pick-up apparatus. Thus, the conventional technology presents obstacles to the current trend of producing ever more compact image pick-up apparatuses.
In an attempt to overcome such obstacles, Korean Patent Publication No. 2003-0001072 discloses an image sensor in which an infrared cut filter is mounted on a micro lens by alternately stacking oxide and nitride layers on the micro lens. Although, the cited invention can shorten a back focal length of the total length by mounting the infrared cut filter on the micro lens, the telecentric angle still increases and creates reddish noise at the center or bluish noise at the edge regions of an image.
To solve this problem, Japanese Patent Laid-Open Publication No. 2002-139605 discloses a lens manufactured using a glass that absorbs near-infrared rays. However, the invention of the Japanese patent is costly to implement and therefore not well suited for mass-produced devices.